This application is based upon and claims the benefit of Japanese Patent Applications No. 2000-115530 filed on Apr. 17, 2000, No. 2000-115531 filed on Apr. 17, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a pressure sensor so constructed that environmental pressure outside a case is conducted to a sensor element within the case through a filter, and particularly to a pressure sensor used under harsh environment in which the pressure sensor easily takes water.
2. Description of the Related Art
JP-A-9-43085 and JP-A-9-43076 propose this kind of pressure sensors. In the pressure sensor, a sensor element for detecting pressure is disposed within a case into which environmental pressure (atmospheric pressure) and pressure to be measured (measurement pressure) can be introduced so that the sensor element detects a differential pressure between the two pressures. A water repellent filter is further disposed in a passage through which environmental pressure is introduced, and removes dusts, moisture, and the like in the air.
When the conventional pressure sensor as described above is attached to a measurement member such as a fuel tank of a vehicle, the filter is positioned horizontally with a filter surface facing downward. Because of this, if moisture within pressure medium (air) is attached to the filter, the moisture is difficult to be dropped due to surface tension thereof and may clog the filter. The clogged filter closes the introduction passage of environmental pressure, and prevents the sensor from working. Especially in a case where the sensor easily takes water of rainwater or the like, the above problem becomes prominent.
The present invention has been made in view of the above problem. An object of the present invention is to prevent clogging of a filter in a pressure sensor so constructed that environmental pressure is introduced into a case through the filter toward a sensor element unit disposed in the case.
According to one aspect of the present invention, briefly, a filter attached to an environmental pressure introduction port, through which environmental pressure is introduced into a case, has a filter surface that is positioned along a gravitational direction when a pressure sensor is used. In the pressure sensor, because the filter surface is positioned along the gravitational direction, moisture attached to the filter surface can easily drop due to the self-weight thereof. Therefore, the filter can be prevented from clogging with moisture. Preferably, the filter surface has a convex shape protruding outwardly from the case. In this case, moisture attached to the filter surface can drop more easily due to reduced surface tension thereof.
According to another aspect of the present invention, an environmental pressure introduction port covered with a filter is divided into a plurality of opening portions. In this case, even little moisture is liable to condense, so that the removal of moisture can be promoted.